That said, if it were me, I would size up a half-size. But YMMV, and my feet are funny.

Thanks I had to return a pair of shoes on the 341 last in size 9.5 as they were 1/2 size too big. I in the process of decided if I want the same shoes in a 9 (I think should fit well) or try another last.

Thanks I had to return a pair of shoes on the 341 last in size 9.5 as they were 1/2 size too big. I in the process of decided if I want the same shoes in a 9 (I think should fit well) or try another last.

If I were you, I would try the same shoes in a 9 (a safer bet). You might end up paying another expensive lesson if trying another last.

Yes, 348 compared to 341 is about 6mm longer (4/16). For the average feet RTW benchgrades coming mostly in E (UK normal) fitting are running true to size, at least in my experience.
I'd say same size in both lasts should be fine for average feet, though former would be somewhat longer than latter.

The 341 can be just a tiny bit narrower at the toe width than the 348. Due to the more rounded configuration of the 341 and it's overall shorter length. I have pairs in both lasts and despite this minor difference find that both fit me well in the same size. The slightly tight toe on the 341 eased up quickly with a few wearings.

Can anyone advise on the difference in fit between the C & J's 341 and 348 last.

My foot fits a 10.5 b or c in Allen Edmonds but is a 9E in the 341 last in Crockett and Jones. I am wondering if the 348 last is narrow enough that I should buy a 9.5E in it.

I did a comparison of an AE shoe in 10.5B (US size) on the 0-Last to a C&J on the 348-last in 8.5E (UK size) and found the C&J to be very similar in width, but a bit smaller in length. Therefore, I would expect a 9.5E (UK size) C&J on 348 to be too big for you.